1. Field of the Invention
The invention relates to a semiconductor wafer processing system, and in particular, to a backside gas delivery system for a semiconductor wafer processing system.
2. Description of the Background Art
Electrostatic chucks (ESC) are used in many single-wafer processing systems for retaining wafers during processing. In order to improve heat conduction between the wafer and the chuck, a backside gas supplied between the wafer and the support surface of the chuck is often used as a heat transfer medium. The design of a backside gas delivery system may have significant impact on the performance of the wafer processing system, especially in cases where stringent pressure control is required. In many existing designs, when the wafer is removed from the chuck upon completion of processing, the backside gas is often allowed to enter the process chamber, resulting in a temporary yet undesirable pressure increase. For certain applications, the increased pumpdown time between wafers may have an adverse impact on the process throughput. An ion implanter, for example, is particularly susceptible to such inadvertent, pressure fluctuations because of the stringent requirement for a low operating pressure, typically in the range of about 10.sup.-6 torr. Any increase in chamber pressure from the backside gas requires additional vacuum pumping and directly results in a decreased throughput. Maintaining a high process throughput is of special concern to a serial ion implanter. Furthermore, a transient pressure increase may result in other adverse effects--e.g., the ion beam may be neutralized, or be deflected onto the side of the flight tube, which may cause contaminants to be sputtered from surfaces within the equipment.
One solution involves a gas delivery system designed to rapidly remove the gas from the wafer backside with limited gas leakage into the process chamber. This is disclosed in a commonly-assigned U.S. patent application Ser. No. 08/154,509 entitled "Backside Gas Quick Dump Apparatus for a Semiconductor Wafer Processing System", filed on May 12, 1997, and is herein incorporated by reference.
However, there is always an ongoing need for alternative backside gas delivery systems that provide a reliable gas shutoff and minimal pressure increase during wafer transfer. Additionally, a gas delivery system having simplified vacuum connections is also desirable.